The present disclosure relates to a lithium ion secondary battery using a lithium composite oxide as a positive electrode active material of a positive electrode, an electric tool, an electric vehicle, and a power storage system which use the lithium ion secondary battery.
In recent years, a small-sized electronic apparatus represented by a portable terminal device or the like has become widespread, and a further reduction in size and weight, and a long operation lifespan are strongly demanded. Along with this, a development of a battery as a power source, particularly, a secondary battery, which is small in size and is light in weight, and which can obtain a high energy density, has been progressed. In recent years, this secondary battery has been reviewed for an application for use in a large-sized electronic apparatus such as a vehicle while not being limited to a small-sized electronic apparatus.
As secondary batteries, secondary batteries using various charge and discharge principles have been widely proposed, but among these, a lithium ion secondary battery using occlusion and emission of lithium ions has attracted attention. This is because an energy density higher than that in a lead battery, a nickel-cadmium battery, or the like, can be obtained.
The lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution, and the positive electrode and the negative electrode include a positive electrode active material and a negative electrode active material that occludes and emits lithium ions, respectively. As the positive electrode active material, a lithium composite oxide including lithium and a lithium transition metal as a constituent element has been widely used. The selection of the positive electrode active material that is directly related to a charge and discharge reaction has a large effect on a battery performance, such that various studies have been undertaken with respect to the composition of the lithium composite oxide.
Specifically, to obtain a large capacity and a high potential, and to improve charge and discharge cycle characteristics, there is proposed a method in which a lithium composite oxide expressed by LiaMIbMIIcOd (MI includes Mn, Ni, Co, or the like, MII includes Al or the like, 1.1<a<1.5, 0.9<b+c<1.1, 1.8<d<2.5) is used (for example, refer to a specification of Japanese Patent No. 3873717). However, a composition ratio (Li/the sum of MI and MII) of Li with respect to the sum of MI and MII is larger than 1 in a mole ratio.
To ameliorate a loss of a positive electrode capacity caused by an irreversible capacity of an Si-based or Sn-based negative electrode active material, and to sufficiently utilize a high capacity characteristic of the negative electrode active material, there is proposed a method in which a lithium-rich lithium composite oxide expressed by LihMniCojNikO2 is used (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-158415). Here, h=[3(1+x)+4a]/3(1+a), i=[3α(1+x)+2a]/3(1+a), j=β(1−x)/(1+a), k=γ(1−x)/(1+a), 0<a<1, α>0, β>0, γ>0, α+β+γ=1, 0≦x<1/3. This composite oxide is a solid solution expressed by Li1+x(MnαCoβNiγ)1-xO2.aLi4/3Mn2/3O2. However, the cycle characteristics are not sufficiently satisfied.